Gas tube overvoltage protectors are widely used for the protection of equipment from overvoltage conditions which may be caused by lighting, high voltage line contact, and the like.
It is also a widely practiced technique to associate various fail-safe arrangements with such tubes and with other types of protectors, e.g., air gap arresters, to meet various contigencies. For example, the presence of a sustained overload, as where a power line has come in continued contact with a protected telephone line, produces a concomitant sustained ionization of the gas tube and the resultant passage of heavy currents through the tube. Such currents will in many cases destroy the overvoltage protector and may also constitute a fire hazard.
One common approach to this problem is to employ fusible elements which fuse in the presence of such overloads and provide either a permanent short circuiting of the arrester directly, or function to release another mechanism, e.g., a spring loaded shorting bar, which provides the short circuit connection (commonly, the arrester electrodes are both shorted and grounded). The presence of the permanent short and ground condition serves to flag attention to that condition thus signalling the need for its inspection or replacement. Examples of this type of fail-safe protection are found in U.S. Pat. Nos. 3,254,179, 3,281,625, 3,340,431, 3,396,343 and 3,522,570. Several of these patents also incorporate with the fail-safe feature, a backup air gap arrangement so that there is both fail-safe fusible (short) type protection as well as backup air gap protection. Copending application Ser. No. 719,076, filed Aug. 31, 1976, entitled "Multi-Function Fail-Safe Arrangements For Overvoltage Gas Tubes," now U.S. Pat. No. 4,062,054, and owned by the assignee of the instant invention, also discloses several related arrangements and is incorporated herein by reference.
A characteristic common in the foregoing arrangements is the employment of metallic materials for the fusible elements. This is an established practice in the art directed by the object of forming from the fused material an electrically conductive shorting path to thereby short circuit the arrester electrodes. The selection is also ordinarily dictated by the well known, widely exploited fusion properties of metals, and by thermal conductivity and related factors involved in fail-safe arrester designs.
The present invention is based on the discoveries that an effective fail-safe function can be achieved by employing a non-metallic fusible material and that important advantages are consequently realized. The fusible material is an electrical insulator which in the exemplary embodiments is interposed between one or more of the electrodes and the shorting mechanism. Surprisingly, the response of the non-metallic material to thermal conditions is precise and, moreover, does not leave an insulative film in the course of fusing which might otherwise interfere with the short circuit contact.
While the patent literature discloses the use of non-metallic fusible materials in certain fail-safe applications (see U.S. Pat. Nos. 1,580,516 and 1,457,249) the arrangements suggested therein involve movement of the arrester electrodes and/or direct arc involvement, as well as other materials, techniques and environmental factors which compromise many of the advantages that applicant has found can be realized from the non-metallic fusion materials disclosed herein.